A static typing system for ruby in ruby

output

$ ruby types_usage.rb
Fermenting the cheese for 5 days!
Fermenting the cheddar for 0.5 days!
./types.rb:7:in `types': Expected #<Walrus:0x7f7e71bb4328> to be a Cheese, but was a Walrus (ArgumentError)
        from types_usage.rb:1:in `each'
        from types_usage.rb:1:in `each_slice'
        from ./types.rb:5:in `each'
        from ./types.rb:5:in `each_slice'
        from ./types.rb:5:in `each'
        from ./types.rb:5:in `types'
        from types_usage.rb:22:in `ferment'
        from types_usage.rb:28

types.rb

class Object
  def types(*args)
    raise ArgumentError.new("types must have an even number of arguments") unless args.length.even?

    args.each_slice(2).each do |type, value|
      unless value.is_a?(type)
        raise ArgumentError.new("Expected #{value.inspect} to be a #{type}, but was a #{value.class}")
      end
    end
  end
end

types_usage.rb

require 'types'

class Cheese
  def name
    "cheese"
  end
end

class Cheddar < Cheese
  def name
    "cheddar"
  end
end

class Walrus
  def name
    "Bill"
  end
end

def ferment(cheese, days)
  types Cheese, cheese,
        Numeric, days
  puts "Fermenting the #{cheese.name} for #{days} days!"
end

ferment Cheese.new, 5
ferment Cheddar.new, 0.5
ferment Walrus.new, 10